Amplifier circuit



March 29, 1966 Filed Oct. 1, 1962 v. N. ERDMAN ET A L AMPLIFIER CIRCUIT 2 Sheets-Sheet 1 [@sm RI b A W n (LI R2 Q2 TM! S1 Ql a2 Tia Tlb m m B 2a T2 NW) MMFNTOES VADIM N. ERDMAN HAROLD r2. MONROE March 29, 1966 v. N. ERDMAN ETAL 3,

AMPLIFIER CIRCUIT Filed Oct. 1, 1962 2 Sheets-Sheet 2 m1 mast mMn/fazs' VADIM MERDMAN AROLD R. MONROE United States Patent 3,243,511 AMPLlFiER CIRCUIT Vadim N. Erdman, Los Angeies, and Harold R. Monroe, Santa Monica, Caiii'l, assignors to Douglas Aircraft Company, The, Santa Monica, Caiif.

Filed Get. 1, 1962, Ser. No. 227,361 9 Claims. (Cl. 1791) Our invention relates generally to communication systems and more particularly to an amplifier circuit which is especially useful for providing voice amplification in protective breathing equipment such as masks.

Protective breathing masks are highly desirable and often critically necessary in fighting fires, for example, where smoke is thick and litle oxygen is available in the surrounding air. The mask must cover at least the eye and nose regions of a fire fighters face in order to protect his eyes and permit normal breathing through the nose. However, where physical exertion is great, as would be the case in fighting a fire, the fire fighter will and must also breathe through his mouth. This means, of course, that the mask should additionally cover the region of the fire fighters mouth so that he will be able to breathe air supplied only through the mask, and is prevented from directly inhaling any smoke by way of his mouth. The difficulty with such masks, however, is that the persons voice becomes mufiied to such an extent as to become virtually inaudible or unintelligible even when the person is shouting.

An essentially expected solution to the problem of speaking while wearing a mask which covers the mouth is, of course, to mount a microphone before the fire fighters month, Within the mask, and provide him with a loudspeaker and an audio amplifier connecting With the microphone. lt is obvious that such equipment should be extremely lightweight and compact so that the fire fighter Will not be burdened or hampered in any manner. Further, it is desirable that the loudspeaker be positioned in the vicinity of the fire fighters mouth and moved in unison with movement of his head, such that the loudspeaker generated sounds are directional. The loudspeaker generated sounds should also be proportional in volume to that of the persons voice, of course.

The fire not only uses up much of the oxygen in the air in the area of the fire but also raises the temperature of the ambient air usually in accordance with its proximity to the body of the fire. Fires also frequently occur during freezing weather, too. The audio amplifier and loudspeaker should, therefore, be able to withstand temperatures varying from very low limits to very high limits and the amplifier circuit, in particular, should be able to perform effectively with stability and without distortion of the loudspeaker reproduced voice over a wide range of temperature variations. This, of course, implies a com pleX circuit with a relatively great number of components together with a large power supply, and which is clearly not consonant with the above requirement for extremely lightweight and compact equipment.

Bearing in mind the foregoing, it is a major object of our invention to provide an amplifier circuit that is extremely lightweight and compact but which performs efficiently and effectively with stability and low distortion in amplifying, for example, the voice of a person.

Another object of the invention is to provide communication means Which is eminently usable with protective breathing masks, such as those worn by fire fighters, Without burdening or hampering the wearer in any manner.

A further object of this invention is to provide an amplifier circuit which can be used with a fire fighters mask, for example, and is operatively stable with low distortion Patented Mar. 29, 1936 ICQ of amplified sounds over a wide range of ambient temperatures.

A still further object of the invention is to provide an amplifier circuit which draws very little current until driven, and a relatively small power supply is required for the circuit.

Briefly, and in general terms, the foregoing and other objects are preferably accomplished by providing a two stage, transformer coupled transistor amplifier circuit having a carbon microphone input and a driven loudspeaker in its output. The two transistor stages are not biased and a feedback network connects the output of the second stage to the input of the first stage. The network provides negative feedback at the higher frequencies to eliminate hiss, and includes a thermistor having a negative temperature coefficient to introduce a resistance which varies inversely with ambient temperature to stabilize the amplifier circuit over a wide range of temperature variations.

Our invention possesses other objects and advantages, some of which together with the foregoing, will become apparent from the following description of an illustrative example of the invention. The invention will be more fully understood by reading the description with joint reference to the attached drawings, in which:

FIGURE 1 is a fragmentary perspective showing a communication system including our amplifier circuit installed and used with a protective breathing mask which is Worn by, for example, a fire fighter;

FIGURE 2 is a circuit diagram of an illustrative embodiment of our amplifier circuit; and

FIGURE 3 is a graph showing a number of curves which illustrate corresponding voltage waveforms at different points of the amplifier circuit of FIGURE 2.

FIGURE 1 shows a fire fighter 10 wearing a smoke mask 12 having a microphone M1 mounted within the mask 12, before the fire fighters mouth, and an amplifier unit 14 attached to the mask 12 directly before and below the fire fighters mouth. The amplifier unit 14 mounts a loudspeaker (SP1) directly behind the grill 14a and includes our amplifier circuit installed within the housing 14b of the unit 14. A toggle switch S1 is located on one side of the housing 1412 just above switch guard 140. A battery (B1) is included within the housing 14b and is located in the normally lower portion thereof. A short cable 16 extends out of the housing 14b andis suitably passed through the mask 12, to connect with the microphone M1. The amplifier unit '14 and its associated components are, of course, properly weatherproofed and sealed.

When the fire fighter 10 wishes to use his communication system, the switch S1 is thrown upwards, closing the switch to connect the systems power supply or' battery (B1) to the amplifier circuit in the housing 141;. The circult is energized and whenever the fire fighter It) speaks, his voice is amplified and reproduced by the loudspeaker (SP1) located behind the grill 14a. The louder the fire fighter 10 speaks, the louder will be the output of the loudspeaker (SP1). The mounting of the amplifier unit 14 to the mask 12 at a location near the fire fighters mouth, directly before and below it, produces amplified sounds which are directional with any movement of the fire fighters head. However, his vision will not be obscured or blocked to any extent by the unit 14.

FIGURE 2 is a circuit diagram of the communication system used with the mask 12. The amplifier circuit is extremely efiicient and efiective in meeting and satisfying the communication needs and requirements of the fire fighter 10. The power output is excellent for the size and complexity of the circuit. This circuit is, of course, easily adapted for uses other than that of a fire fighters mask, and is not to be considered as being limited to any particular field or application. a

The microphone M1 is preferably a carbon microphone which has lead 16a connected to the positive terminal of the battery B1, and another lead 16b connected to one end of the loading resistor R1. The other end of the resistor R1 is connected to the pole of switch S1, the switch pole being engageable with a contact which is connected to the negative terminal of the battery B1. The resistance of the resistor R1 is approximately equal to the internal alternating current impedance of the microphone M1. Thus, the resistor R1 and microphone M1 form a voltage divider across which is applied the voltage of the battery B1 when switch S1 is closed.

The resistance of the microphone M1 is varied inversely with the input to the microphone, such that the voltage across the microphone M1 will be varied with the voice input thereto. microphone M1 therefore produces a proportionately large amplitude alternating voltage which is applied to the primary winding Tla of transformer T1, the primary winding Tla being connected on one end to lead 1612 through blocking capacitor C1 and to lead 166; on the other end.

The secondary winding Tlb of the transformer T1 is connected on one end directly to the base of transistor Q1,- and the other end of the secondary winding Tlb is connected directly to lead 16a. The polarities of the transformer T1 windings Tla and Tlb are such that a negative voltage on the primary winding Tla produces a negative voltage on the base of the transistor Q1 so that there is conduction through it. The emitter of the transistor Q1 is connected to lead 16a, and the collector is connected to the pole of switch S1 through the primary winding T2a of transformer T2. Thus, the transistor T1 does not conduct on positive half cycles of the alternating voltage applied to the primary winding Tla of the transformer T1.

The resistance of the microphone M1 is approximately 470 ohms and the resistance of the primary winding Tla of the transformer T1 is 50 ohms, for example. The resistance of the secondary winding Tlb is, in this example, 1000 ohms. In view of the relatively higher resistance of the microphone M1, it would be expected that the higher resistance winding Tlb should be connected to the microphone through the capacitor C1, and the lowerresistance winding Tla should be connected to the base of the transistor Q1. In other words, under normal practice the resistances of primary and secondary windings Tla and Tl b should be interchanged. Note, also, that no bias is provided for the first stage transistor Q1, which omission is not normally considered feasible for voice or audio amplification.

These same conditions apply to the windings T2a and T21) of transformer T2, and the second stage transistor Q2. One end of the secondary winding T21: is connected directly to the base of the transistor Q2, and the other end is connected directly to lead 16a. No bias is provided for the second stage transistor Q2, and the primary winding T2a which is connected in the high resistance collector circuit of transistor Q1, has a relatively low resistance of, for example, 50 ohms Whereas the resistance of the secondary winding-T212 connected in the lowresistance base circuit, has a high resistance of 1000 ohms. Yet, the efficiency and low distortion of the amplifier circuit is remarkably good in comparison to circuits of similar complexity. A circuit having com A high volume input to the g to the amplitude of the input signal to the transformer T1. The impedance of the loudspeaker SP1 is low compared to that of the transistor Q2 such that it is essentially a current device which is not affected appreciably to any extent by potential variations of the collector of transistor Q2. a

The potential of the collector of the transistor Q2 rises positively, of course, with conduction and this positive voltage is fed back to the base of the transistor Q1 through a feedback network 18 including a series combination of a thermistor TM1, capacitor C2, and resistor R2. It will be recalled that the base of the conducting transistor Q1 was negatively energized so that the network 18 is an opposing or negative feedback network. Parasitic oscillations are damped and good stability of the amplifier circuit results.

The thermistor TM1 has a negative temperature coefiicient, and the values of the capacitor C2 and resistor R2 are selected to provide a series resonant feedback network 18 which eliminates the higher harmonics by negative feedback. For example, frequencies from 800 to 1000 c.p.s. approximately are fed back to cancel such frequencies appearing at the input to the first stage parable performance would'require far more components of the amplifier circuit. This eliminates any hiss or whistling from the system.

The negative feedback network 18 also acts to stabilize the amplifier circuit over a wide range of ambient temperatures. The thermistor TM1 is variably effective for approximate temperatures from below freezing of 30 F. to F., for example. As the ambient temperature increases near a fire, the base current of a transistor increases with the temperature to increase conduction through the transistor. The amplifier circuit would thus be caused to oscillate at higher ambient temperatures without the feedback network 18. However, since the thermistor TM1 drops in resistance with increasing temperature, a larger amount of feedback is provided by the feedback network 18 to damp out any oscillations which may tend to be started.

The transformers T1 and T2 are connected as voltage step-up devices and act as voltage amplifiers, and the transistors Q1 and Q2 are connected :to act as current amplifiers. Sinceno bias is provided for the transistors Q1 and Q2, the amplifier circuit draws very little current (17 ma. from a 9.6 volt battery) until driven (consuming up to 300 ma. average and 800 ma. peak current). Such Class B operation of the transistors Q1 and Q2 produces a high power output and high efficiency circuit but generally introduces harmonic distortion in the output which, however, has decreasing effect as the input signal increases. Thus, the harmonic distortion contributes little disturbing effects on the output signal for high input signal conditions.

As the input signal increases, the efliciency of the amplifier circuit also goes up with the increase of power output. Now, at low signal values, it would appear that the harmonic distortion may rise to relatively intolerable levels. This was not found to be the case, however, for this circuit in actual use. While the transistors Q1 and 2 only conduct on negative half cycles of an alternating voltage applied to the primary winding Tla of the transformer T1, the relatively low base input impedance of the transistors load the amplified output voltages of the transformers T1 and T2 such that the amplified signals are made more symmetrical, with reasonably similarly shaped peaks and troughs so that even harmonics tend to be suppressed and the odd harmonics prevail. Further, at low input levels, the voice of a person has naturally less of the frequencies which create disturbing harmonic distortion.

The emitter. of transistor Q2 is connected to lead 161:, and the collector is connected to the pole of switch S1 throughthe series connected loudspeaker SP1 and through the switch to the negative terminal of the battery B l. The negative terminal of the battery B1 is connected to the center contact of jack Jtl through series resistor R3, and the positive terminal of battery B1 is connected to the shell contact of the jack Ill. The battery B1 is, for example, a nickel-cadmium 180 milliampere-hour rechargeable battery, and can be recharged over 5000 times through the jack J 1 connection. The switch S1, of course,

must be open when the battery B1 is being charged. The

battery Bi supplies sufficient power for actual voice amplification of continuous talking with normal loudness for one hour at 180 ma. average current. More than 8 hours of communicating may be obtained when speaking 10 percent of the time, before recharging of the battery B1 s required.

A highly satisfactory circuit has been obtained by use of the following components and values in the circuit shown in FIGURE 2.

Bl 9.6 v., 180 ma.-hour,

Gulton Industries, Inc. M1 MC 254-A,

Western Electric SP1 27AO6Z12,

Quam Nickols C0. C1 2 mid. C2 .05 mfd.

R1 470 ohms R2 2.7 kilohms R3 39 ohms Q1, Q2 2N270 (or 2Nl374) TMl 35 D2,

Victory Engineering T1, T2 580-12,

United Transformer Corp.

FIGURE 3 is a graph which shows a number of curves drawn from actual oscilloscope (mirror image) photographs of the voltage waveforms respectively at the points A through G as indicated in FIGURE 2. Since mirror image prints were copied, time is to be construed as increasing from right to left. The curves are labeled with corresponding letters with primes in FIGURE 3 and serve to illustrate the actual operation of our amplifier circuit in detail. A sine wave signal A at the corresponding point A of the amplifier circuit produces a signal at the point B having a Waveform as illustrated by curve B.

There is some phase shift due to the capacitor 01 and primary winding Tla, and the negative portions of the waveform of curve B are seen to be of less amplitude than its positive portions due to the loading effect of the low base impedance of transistor Q1 being reflected into the primary winding Tda. The curve B has been drawn to a larger scale than that for the curve A in order to illustrate the Waveform of the curve B more clearly.

Curve C clearly shows the loading effect of the transistor Q1 during the negative portions of the curve. Curve D is essentially an amplified and inverted version of curve C including the inductive effects of the primary winding T242 of the transformer T2. Curve E is, of course, similar to curve D amplified and inverted, and further loaded during the negative portions by the low base input impedance of the transistor Q2.

Remembering that the positive high voltage spikes o f the curve E does not affect the current conduction of the transistor Q2 to any extent after it is cut off, the fairly symmetrical curve F is produced at the collector point P of the transistor Q2. Note that when the waveform of curve F is positive, the waveform of curve C is negative for proper negative feedback action in the circuit.

The curve G at the point G shows the alternating voltage which is superimposed on the direct voltage of the battery B1. The curve G is similar to the curve F. Since an alternating voltage appears at point G, there is some positive feedback effect caused by the signal illustrated by curve G on the amplifier circuit through resistor R1 and the primary winding T2a of transformer T2. This positive feedback would cause oscillation in the 6 amplifier circuit except for the action of the negative feedback network 18.

While some specific components and values for the illustrative embodiment of our invention have been given above, it is to be understood that these specific components and values are exemplary only, and that the particular embodiment of the invention described above and shown in the drawings is merely illustrative of, and not restrictive on our broad invention, and that various changes in design, structure and arrangement may be made without departing from the spirit and scope of the appended claims.

We claim:

1. An amplifier circuit, comprising:

a microphone;

a loading resistor connected in series with said microphone in a first series combination;

a direct current power supply connectable to said first series combination for energizing the same;

a first transformer including a primary winding, and a secondary winding;

a blocking capacitor connecting the common junction between said loading resistor and microphone of said first series combination to one end of said primary Winding of said first transformer, the other end of said primary of said first transformer being connected to the other end of said microphone;

a first transistor including a base, collector and emitter,

said base being connected to one end of said secondary Winding of said first transformer and said emitter and the other end of said secondary winding of said first transformer being connected to said other end of said microphone, and said one end of said primary winding of said first transformer connecting with said blocking capacitor being of similar polarity as said one end of said secondary winding of said first transtformer connecting with said base of said first transistor;

a second transformer including a primary Winding, and

a secondary winding, said collector of said first transistor being connected to one end of said primary winding of said second transformer in a second series combination including said first transistor and said primary winding of said second transformer, said direct current power supply being connectable to said second series combination for energizing the same;

a second transistor including a base, collector and emitter, said base being connected to one end of said secondary Winding of said second transformer and said emitter and the other end of said secondary winding of said second transformer being connected to said other end of said microphone, and said one end of said primary winding of said second transformer connecting with said collector of said first transistor being of opposite polarity as said one end of said secondary Winding of said second transformer connecting with said base of said second transistor;

a loudspeaker, said collector of said second transistor being connected to one end of said loudspeaker in a third series combination including said second transistor and said loudspeaker, said direct current power supply being connectable to said third series combination for energizing the same; and

a negative feedback network including a feedback capacitor and a feedback resistor connected in a fourth series combination connecting said collector of said second transistor to said base of said first transistor, said feedback network being resonant to eliminate a predetermined range of frequencies from said amplifier circuit by negative feedback.

2. An amplifier circuit, comprising:

a microphone;

a loading resistor connected in series with said microphone in a first series combination;

a direct current power supply connectable to said first series combination for energizing the same;

a first transformer including a primary Winding, and a secondary winding;

a blocking capacitor connecting the common junction between said loading resistor and microphone of said first series combination to one end of said primary Winding of said first transformer, the other end of said primary vwinding of said first transformer being connected to the otherend of said microphone; 7 V

a first transistor including a base, collector and emitter,

said base being connected to one end of said secondary winding of said first transformer and said emitter and the other end of said secondary winding of said first transformer being connected to said other end of said microphone, and said one end of said primary winding of said first transformer connecting with said blocking capacitor being of similar polarity as said one end of said secondary winding of said first transformer connecting with said base of said first transistor;

a second transformer including a primary winding, and

a secondary winding, said collector of said first transistor being connected to one end of said primary Winding of said second transformer in a second series combination including said first transistor and said primary winding of said second transformer, said direct current power supply being connectable to said second series combination for energizing the same;

a second transistor including a base, collectorand emitter, said base being connected to one end of said secondary Winding of said second transformer and said emitter and the other end of said secondary Winding of said second transformer being connected to said other end of said microphone, and said one end of said primary winding of said second transformer connecting with said collector of said first transistor being of opposite polarity as said one end of said secondary winding of said second transformer connecting with said base of said second transistor;

a loudspeaker, said collector of said second transistor being connected to one end of said loudspeaker in a third series combination including said second transistor and said loudspeaker, said direct current power supply being connectable to said third series combination for energizing the same; and

a negative feedback network including a negative temperature coefficient element, a feedback capacitor and a feedback resistor connected in a fourth series combination connecting said collector of said second transistor to said base of said first transistor, said feedback network being resonant to eliminate a predetermined range of frequencies by negative feedback and acting to stabilize said amplifier circuit over a predetermined range of ambient temperatures.

3. An amplifier circuit, comprising:

a microphone for receiving an audio input thereto;

a loading resistor connected in series with said microphone in a first series combination;

a direct current power supply connectable to said first series combination for energizing the same;

a first voltage step-up transformer including a primary Winding, and a secondary winding;

a blocking capacitor connecting the common junction between said loading resistor and microphone of said first series combination to one end of said primary emitter and the other end of said secondary winding of said first transformer being connected to said other end of said microphone, and said one end of said primary winding of said first transformer connecting With said blocking capacitor being of similar polarity as said one end of said secondary winding of said first transformer connecting with said base of said first transistor; V v

a second voltage step-up transformer including a primary winding, and a secondary Winding, said collector of said first transistor being connected to one end of said primary winding of said second transformer in a second series combination including said first transistor and said primary winding of said second transformer, saiddirect current power supply being connectable to said second series combination for energizing the same;

a second transistor including a base, collector and emitter, said base being connected to one end of said secondary winding of said second transformer and said emitter and the other end of said secondary winding of said second transformer being connected to said other end of said microphone, and said one end of said primary winding of said second transformer connecting with said collector "of said first transistor being of opposite polarity as said one end of said secondary winding of and second transformer connecting with said base of' s'aid second transistor; i i

a loudspeaker, said collector of said second transistor being connected to one .end of said loudspeaker in a third series combination including said second transistor and said loudspeaker, said direct current power supply being connectable to said third series combination for energizing the same; and

a negative feedback network including a negative temperature coefiicient element, a feedback capacitor and a feedback resistor connected in a fourth series combination connecting said collector of said second transistor to said base of said first transistor, said feedback network being resonant to eliminate a predetermined range of frequencies by negative feedback and acting to stabilize said amplifier circuit over a predetermined range of ambient temperatures.

4. An amplifier circuit, comprising:

a microphone for receiving an audio input thereto;

a loading-resistor-connected in series with said microphone in' a first series combination;

a direct current power supply connectable to said first series combination for energizing the same;

a first voltage step-up transformer including a primary winding of relatively low impedance, and a secondary winding of relatively high impedance;

a blocking capacitor connecting the common junction between said loading resistor and microphone of said first series combination to one end of said primary winding of said first transformer, the other end of i said primary winding of said first transformer being connected to the other end of said microphone;

a first transistor including a base, collector and emitter,

said base being connected to one end of said secondary winding of said first transformer and said emitter and the other end of said secondary winding of said first transformer being connected to said other end of said microphone, and said one end of said primary winding of said first transformer con: necting with said blocking capacitor being of similar polarity as said one end of said secondary winding of said first transformer connecting with said base of said first transistor;

a second voltage step-up transformer including a primary winding of a relativelytlow impedance, and a secondary Winding of relatively high impedance, said collector of said first transistor being connected to one end of said primary Winding of said second transformer in a second series combination including said first transistor and said primary winding of said second transformer, said direct current power supply being connectable to said second series combination for energizing the same;

second transistor including a base, collector and emitter, said base being connected to one end of said secondary winding of said second transformer and said emitter and the other end of said secondary winding of said second transformer being connected to said other end of said microphone, and said one end of said primary Winding of said second transformer connecting with said collector of said first transistOr being of opposite polarity as said one end of said secondary winding of said second transformer connecting with said base of said second transistor; loudspeaker, said collector of said second transistor being connected to one end of said loudspeaker in a third series combination including said second transistor and said loudspeaker, said direct current power supply being connectable to said third series combination for energizing the same; and

a negative feedback network including a negative tema loading resistor connected in series with said microphone in a first series combination, said microphone having an intern-a1 alternating current impedance substantially equal to the resistance of said loading resistor;

direct current power supply connectable to said first series combination for energizing the same;

a first voltage step-up transformer including a primary winding of relatively low impedance, and a secondary Winding of relatively high impedance; blocking capacitor connecting the common junction between said loading resistor and microphone of said first series combination to one end of said primary winding of said first transformer, the other end of said primary winding of said first transformer being connected to the other end of said microphone;

a first transistor including a base, collector and emitter,

said base being connected to one end of said secondary winding of said first transformer and said emitter and the other end of said secondary winding of said first transformer being connected to said other end of said microphone, and said one end of said primary winding of said first transformer connecting with said blocking capacitor being of similar polarity as said one end of said secondary winding of said first transformer connecting with said base of said first transistor;

second voltage step-up transformer including a primary Winding of relatively low impedance, and a secondary winding of relatively high impedance, said collector of said first transistor being connected -to one end of said primary Winding of said second transformer in a second series combination including said first transistor and said primary winding of said second transformer, said direct current power supply being connectable to said second series combination for energizing the same;

a second transistor including a base, collector and emitter, said base being connected to one end of said secondary winding of said second transformer and said emitter and the other end of said secondary Winding of said second transformer being connected to said other end of said microphone, and said one end of said primary winding of said second transformer connecting with said collector of said first transistor being of opposite polarity as said one end of said secondary winding of said second transformer connecting with said base of said second transistor;

relatively low impedance loudspeaker, said collector of said second transistor being connected to one end of said loudspeaker in a third series combination including said second transistor and said loudspeaker, said direct current power supply being connectable to said third series combination for energizing the same; and

a negative feedback network including a negative temperature coefiicient thermistor, a feedback capacitor and a feedback resistor connected in a fourth series combination connecting said collect-or of said second transistor to said base of said first transistor, said feedback network being resonant to eliminate a predetermined range of frequencies by negative feedback and acting to stabilize said amplifier circuit over a predetermined range of ambient temperatures.

6. An amplifier circuit adapted for voice amplification in masks and the like, comprising:

carbon microphone mounted within a mask for receiving an audio input thereto;

loading resistor connected in series with said microphone in a first series combination, said microphone having an internal alternating current impedance substantially equal to the resistance of said loading resistor;

a battery connectable to said first series combination for a first transistor including a base, collector and emitter,

Said base being connected to one end of said secondary winding of said first transformer and said emitter and the other end of said secondary winding of said first transformer being connected to said other end of said microphone, and said one end of said primary winding of said first transformer connecting with said blocking capacitor being of similar polarity as said one end of said secondary winding of said first transformer connecting with said base of said first transistor;

second voltage step-up transformer including a primary winding of relatively low impedance, and a secondary Winding of relatively high impedance, said collector of said first transistor being connected to one end of said primary winding of said second transformer in a second series combination including said first transistor and said primary winding of said second transformer, said battery being connectable to said second series combination for energizing the same;

a second transistor including a base, collector and emitter, said base being connected to one end of said secondary winding of said second transformer and said emitter and the other end of said secondary winding of said second transformer being connected to said other end of said microphone, and said one end of said primary winding of said second transformer connecting with said collector of said first transistor being of opposite polarity as said one end of said secondary Winding of said second transformer connecting with said base of said second transistor;

a relatively low impedance loudspeaker mounted outside said mask generally' before and below said microphone, said collector of said second transistor being connected to one end of said loudspeaker in a third series combination including said second transistor and said loudspeaker, said battery being connectable to said third series combination for energizing the same; and

a negative feedback network including a negative temperature coeificient thermistor, a feedback capacitor and a feedback resistor connected in a fourth series combination connecting said collector of said second transistor to said base of said first transistor, said feedback network being resonant to eliminate a predetermined range of frequencies by negative feedback and acting to stabilize said amplifier circuit over a predetermined range of ambient temperatures.

7. An amplifier circuit as defined in claim 6 including means for connecting a battery charger to said battery,

8. An amplifier circuit, comprising:

sound transducer means for converting sound into an electrical signal;

a first transformer including a primary winding, and a secondary winding;

means connecting the electrical signal to said primary winding of said first transformer;

a first amplifying transistor including an input having a relatively low impedance and an output having a relatively high impedance, said input of said first amplifying transistor being connected to said secondary winding of said first transformer;

a second transformer including a primary Winding connecting with said output of said first amplifying transistor and a secondary winding, said primary windings of said first and second transformers being of relatively low impedance and said secondary windings of said first and second transformers being of relatively high impedance;

a second amplifying transistor including an input having a relatively low impedance and an output having a relatively high impedance, said input of said second amplifying transistor being connected to said secondary winding of said second transformer and said first and second amplifying transistors being generally operated class B with zero bias applied thereto and zero idle current drawn;

a feedback network connecting said output of said second amplifying transistor to said input of said first amplifying transistor; and

sound reproducing means connecting with said output of said second amplifying transistor.

9. An amplifier circuit as defined intclaim 8 wherein said feedback network is a negative feedback network including a thermistor, feedback capacitor and feedback resistor connected in a series combination.

References Cited by the Examiner ROBERT H. ROSE, Primary Examiner.

WILLIAM C. COOPER, Examiner.

A. SANTORELLI, R. MURRAY, Assistant Examiners. 

1. AN AMPLIFIER CIRCUIT, COMPRISING: A MICROPHONE; A LOADING RESISTOR CONNECTED IN SREIES WITH SAID MICROPHONE IN A FIRST SERIES COMBINATION; A DIRECT CURRENT POWER SUPPLY CONNECTABLE TO SAID FIRST SERIES COMBINATION FOR ENERGIZING THE SAME; A FIRST TRANSFORMER INCLUDING A PRIMARY WINDING, AND A SECONDARY WINDING; A BLOCKING CAPACITOR CONNECTING THE COMMON JUNCTION BETWEEN SAID LOADING RESISTOR AND MICROPHONE OF SAID FIRST SERIES COMBINATION TO ONE END OF SAID PRIMARY WINDING OF SAID FIRST TRANSFORMER, THE OTHER END OF SAID PRIMARY OF SAID FIRST TRANSFORMER BEING CONNECTED TO THE OTHER END OF SAID MICROPHONE; A FIRST TRANSISTOR INCLUDING A BASE, COLLECTOR AND EMITTER, SAID BASE BEING CONNECTED TO ONE END OF SAID SECONDARY WINDING OF SAID FIRST TRANSFORMER AND SAID EMITTER AND THE OTHER END OF SAID SECONDARY WINDING OF SAID FIRST TRANSFORMER BEING CONNECTED TO SAID OTHER END OF SAID MICROPHONE, AND SAID ONE END OF SAID PRIMARY WINDING OF SAID FIRST TRANSFORMER CONNECTING WITH SAID BLOCKING CAPACITOR BEING OF SIMILAR POLARITY AS SAID ONE END OF SAID SECONDARY WINDING OF SAID FIRST TRANSFORMER CONNECTING WITH SAID BASE OF SAID FIRST TRANSISTOR; A SECOND TRANSFORMER INCLUDING A PRIMARY WINDING, AND A SECONDARY WINDING, SAID COLLECTOR OF SAID FIRST TRANSISTOR BEING CONNECTED TO ONE END OF SAID PRIMARY WINDING OF SAID SECOND TRANSFORMER IN A SECOND SERIES COMBINATION INCLUDING SAID FIRST TRANSISTOR AND SAID PRIMARY WINDING OF SAID SECOND TRANSFORMER, SAID 